The present invention relates generally to circuitry for driving discharge lamps and, in particular, to a liquid crystal display (LCD) backlight inverter.
There has been an ever-increasing demand for LCD displays within the past few years. Such displays are being employed by all types of computer devices including flat display monitors, personal wireless devices and organizers, and large public display boards. Typically, LCD panels utilize a backlighting arrangement which includes a discharge lamp that provides light to the displayed images. Among those currently available discharge lamps, cold cathode fluorescent lamps (CCFLs) provide the highest efficiency for backlighting the display. These CCFLs require high voltage AC to operate, mandating a highest efficient DC to AC inverter.
FIG. 1 illustrates a simplified schematic diagram of a conventional LCD backlight inverter 100. As shown in FIG. 1, a well-known Royer circuit 110 is employed to convert a relative low direct current (DC) input voltage into a higher alternating current (AC) output voltage for driving a CCFL 102. The Royer circuit 110 includes a pair of transistors Q11 and Q12, a step-up transformer T1, and a resonant capacitor C11. The capacitor C11 is connected across a primary winding WP of the transformer T1. A secondary winding WS of the transformer T1 is coupled to a ballast capacitor C12 in series with the lamp 102. The transistors Q11 and Q12 are switched on and off alternately by the base drive provided by a feedback winding WF of the transformer T1. In addition, the primary winding WP is provided with a center tap coupled to a buck inductor L1. A DC input source VDC is applied to a transistor-type switch Q13. The inductor L1 coupled between the switch Q13 and the primary winding""s center tap converts input DC voltage to a DC current. A diode D11 connected between the output of the switch Q13 and ground places fixed limit on the voltage excursion across the inductor L1.
Still referring to FIG. 1, the backlight inverter 100 also includes a PWM circuit 120 for dimming control of the lamp 102. Since a lamp""s intensity (lumen) is a direct function of the lamp current, the LCD backlight can be dim-controlled by regulating the lamp current flowing through the CCFL 102. Typically, the lamp current is sensed with a resistor R1 in series with one lead of the lamp 102 and regulated by varying the average voltage impressed across the inductor L1. The PWM circuit 120 detects a sensing signal from a feedback network formed by the resistor R1 and a diode D12, and it also receives a brightness control signal BR with variable DC levels so as to provide a pulse width modulation (PWM) signal to the switch Q13. A LCD panel controller (not shown) generally produces the signal BR with a DC level indicative of the desired amount of current through the lamp. As a result, the PWM circuit 120 changes the duty cycle of its PWM output signal applied to the switch Q13 in response to the feedback sensing signal and the brightness control signal BR. This allows the transistor switch Q13 to vary the average voltage impressed across the buck inductor L1, thereby adjusting the lamp""s current and dimming the CCFL 102.
However, a drawback of the conventional inverter 100 is that dimming control is acquired at the expense of the PWM circuit 120 and the added feedback network, and consequently at higher component count and cost. Especially, the PWM circuit 120 makes up most of the cost of production of the LCD backlight inverter. Therefore, what is needed is an apparatus for dimming control of LCD backlight without the use of PWM circuitry.
It is an object of the present invention to provide an apparatus for driving a discharge lamp that is less costly and includes fewer parts than conventional design.
It is another object of the present invention to provide an apparatus for dimming control of LCD backlight without the use of PWM circuitry.
The present invention is generally directed to an apparatus for driving a discharge lamp. According to one aspect of the invention, the apparatus includes a switching regulator, a DC-to-AC inverter, and level shifter circuitry. The switching regulator receives a DC voltage source and produces a low voltage DC signal, and has a switch configured to turn on and off periodically in response to a duty cycle of a dimming control signal to chop up the DC voltage source output. The switching regulator is therefore used to regulate an average magnitude of the low voltage DC signal. The level shifter circuitry is provided for translating the dimming control signal to a voltage level required for turning on the switch. The DC-to-AC inverter is configured to step up the low voltage DC signal to a high voltage AC signal applied to the discharge lamp, in which the high voltage AC signal provides a lamp current flowing through the discharge lamp. Note that the duty cycle of the dimming control signal is varied according to a brightness table of the relationship between the duty cycle and the lamp current. Further, the inventive apparatus includes a brightness controller having the brightness table of the relationship between the duty cycle of the dimming control signal and the lamp current. The brightness controller generates the dimming control signal and varies the duty cycle of the dimming control signal based on the corresponding lamp current in the brightness table.
According to another aspect of the invention, an apparatus for dimming control of a discharge lamp is disclosed. The inventive apparatus includes a switching regulator receiving a DC voltage source and producing a low voltage DC signal. The switching regulator has a power switch configured to turn on and off periodically in response to a duty cycle of a dimming control signal to chop up the DC voltage source output, and it is used to regulate an average magnitude of the low voltage DC signal. A DC-to-AC inverter is provided for stepping up the low voltage DC signal to a high voltage AC signal applied to a discharge lamp, in which the high voltage AC signal provides a lamp current flowing through the discharge lamp. The inventive apparatus also includes a brightness controller having a brightness table of the relationship between the duty cycle of the dimming control signal and the lamp current. The brightness controller generates the dimming control signal as output and varies the duty cycle of the dimming control signal based on the corresponding lamp current in the brightness table. Moreover, level shifter circuitry coupled between an output terminal of the brightness controller and a control terminal of the power switch is used to translate the dimming control signal to a voltage level required for turning on the power switch.